Contribution description

This PR models the FEATURES_PROVIDED as Kconfig symbols. The idea is, similarly to the current Makefile.features, to select the provided features at every level (i.e. board, CPU model, CPU series, etc.).

In cpu/Kconfig and board/Kconfig I declared common CPU and board symbols, which take their value depending on the selected CPU model and board respectively. Here I proposed to have BOARD, CPU_MODEL, CPU_SERIES, CPU_FAMILY and CPU_ARCH (more specific to less specific), plus CPU which matches the current CPU variable. We should evaluate if that cover our use cases properly.

The selection of symbols starts with the board and goes like:

BOARD_<board> --[select]--> CPU_MODEL_<model> --[select]--> CPU_SERIES_<series> --[select]--> CPU_FAMILY_<family> --[select]--> CPU_ARCH_<arch>

Although this chain of symbols tries to mirror the common symbols, nothing prevents use from using other intermediate symbols (e.g. here I added CPU_ARCH_CORTEX_M0PLUS which selects CPU_ARCH_CORTEX_M).

Done in #13715.

On features

I added cpu/Kconfig.features for the declaration of common feature-like symbols. For features specific to a CPU, board or architecture we can declare them directly on the corresponding Kconfig file. Here I used the HAS_ prefix to indicate features, mostly because it is customary in Kconfig files across projects, but we could use something else.

Done in #14151.

In the future, we could also indicate via this mechanism that, for instance, the CPU_MODEL_SAMD21G18A presents an at86rf233 radio.

Testing procedure

I added the debug-features-provided-kconfig target in dependencies_debug.inc.mk, which should print the same list as info-features-provided, and will allow us to test the migration process. To test it, make sure you set SHOULD_RUN_KCONFIG=1.

Also, one can run make menuconfig and either enable the 'show-all mode' (by pressing a) or search for a particular feature symbol (by presing /), like HAS_PERIPH_GPIO. Once the symbol is selected, one can see more information on the symbol (by pressing ?), like the description of the feature and who selects (provides) it (the board, the cpu model, etc.).

Edit: A test tests/kconfig_features has been added to check that boards provide the same features via Kconfig and via Makefile.features. This is useful during migration process to ensure sync.

Issues/PRs references

Depends on #14151